1. Field of the Invention
This invention relates to a surrounding structure of a probe card, and more particularly to an isolator unit, which is formed by filling in a wire leading region with epoxy material to substitute for an originally clear portion, so as to improve high impedance characteristics of the cross-over portion by using the resultant parasitic capacitance effect, and improve the compensation for the purpose of impedance matching.
2. Description of the Prior Art
After the completion of wafer manufacturing, a wafer test is required to guarantee the functions of the products. The wafer test is performed by using a test device and a probe card to test each die on a wafer, in order to confirm that the electrical characteristics and performances of the die are in accordance with the design specifications. As chip function becomes more sophisticated, the high-speed and accurate test requirement is more significant.
In the circumstance, a probe card is used for the wafer test of semiconductor integrated circuits, which is very critical especially in the test stage of RF (radio frequency) wafer-level mass production. The application is to perform functional tests on bare die by the probe before packaging of integrated circuits, to screen out the defectives and then proceed to further packaging fabrication.
The test process begins with locating a die on a wafer on the test device. The probe card is mounted on the test device, so as to have the contact pads on the die aligned and touched with the probe.
Reference is made to FIGS. 1 and 2, which are respectively top and cross-sectional views of a commercially available epoxy probe card. A probe card 9 comprises a circuit board 92, and hundreds of probes 90. The circuit board 92 has an isolation locking ring 94 which is extending along the lower surface thereof. The probes 90 are fixed on the isolation locking ring 94 of the probe card 9 by means of epoxy element 96. The probes 90 are electrically connected to the circuit board 92, and electrically connected individually to the metal pads 84 on the die 82 of a wafer. The probes 90 are used to contact the pads 84 on the die 82, so as to directly input signals or read the output values to or from the die 82.
During the one-by-one detection of wafer testing, if any die 82 does not pass the test, it is marked as defective. Consequently, during chip dicing and separation, these dies which are marked as defective are screened out and excluded from further packaging fabrication. As a result of the wafer test, the dies that pass the test proceed to the next stage of packaging fabrication.
However, in the design of a conventional epoxy probe card 9, since the wiring portion is far away from the ground wire 93 (distributed over the surface of the circuit board 92), the loop inductance produced becomes large and the problem of impedance mismatching becomes worse. Meanwhile, the dielectric medium 98 between the signal wire 91 and the ground wire 93 is air, the electrical field dissipates in the air, and thus power consumption becomes large when the frequency is increased.
Reference is made to FIG. 3, which is a frequency response graph of a simulation test for the conventional epoxy probe card; FIG. 3A is a frequency response comparison table of S11 and S21 against 3 dB. The simulation test makes comparison between two cases; Case A is a case where the signal wire is far away from the ground wire and Case B is a case where the signal wire is near the ground wire. S11 illustrates the fact that test signals are rebounded due to impedance mismatching, and a value thereof is closer to minus infinite dB is better. S21 illustrates the fact that all testing signals completely pass without any loss, and a value thereof is closer to 0 dB is better. The values are obtained in dB corresponding to the frequency response S11 and S21 of the simulation test to give the graph of FIG. 3. From the graph, it can be found that once the leading wire crosses over that wire, the corresponding return loss decreases with the increase of the frequency, thus resulting in serious signal reflection. In a high frequency band, the corresponding insertion loss will become large; the frequency width corresponding to 3 dB is only about 560–720 MHz. With this structure, under high frequency, the signal loss is quite serious, such that no energy is delivered. In particular, as the speeds of electrical products are increasingly refreshed and there is increased need for high speed, this condition more urgently requires improvement.
Accordingly, there is a need to improve the above inconvenience and disadvantage of the conventional probe card in the actual application.
Accordingly, this invention is provided to improve the above disadvantages with a reasonable design.